There are many industrial applications where it is important to accurately measure physical characteristics of thin films. Fabrication of integrated circuits and flat panel displays are two examples. Often it is necessary to measure the thickness, and optical constants n and k of thin films deposited on a substrate.
One known technique for determining many physical characteristics of thin films and materials is disclosed in U.S. Pat. No. 4,905,170 to Forouhi et al. In this technique, and many others, it is necessary to perform accurate, broadband reflectance and/or transmission measurements of the thin films. When performing such measurements, chromatic distortions can lead to inaccurate results. This happens, for example, if a chromatic response (attenuation as a function of wavelength) of the optical system performing the measurements changes between measurements. An example of a change in chromatic response is when an attenuation of a certain wavelength changes more than an attenuation of another wavelength. Any optical system used must have a fixed chromatic response. Systems that use lenses are generally not suitable because the chromatic response of lens-based systems can be altered by very small mechanical misalignments.
U.S. Pat. No. 5,880,831 to Dale H. Buermann, Abdul Rahim Forouhi, and Michael J. Mandella discloses optical systems suitable for use with thin film characterization methods requiring constant chromatic response. The system uses toroidal mirrors. Small misalignments of the surface being analyzed or the mirrors results in only small changes in chromatic response. This property makes mirrors particularly well suited for use in systems requiring fixed chromatic response.
A shortcoming of the systems disclosed in U.S. Pat. No. 5,880,831 is that free-space light propagation is required. This reduces the mechanical flexibility of the system and constrains the system design. For example, if a spectrophotometer is used, it must have a fixed location with respect to the mirrors. Movement of the system to measure different points on a substrate can therefore be problematic. Another shortcoming is that it is often desirable to perform simultaneous reflectance and transmittance (R&T) measurements at the same location. U.S. Pat. No. 5,880,831 does not teach a system for performing simultaneous R&T measurements.